The subject matter of the invention is lithium-bisoxalatoborate, Li[(C2O4)2B], two methods for the production thereof, and the use of lithium-bisoxalatoborate as a conducting salt in lithium ion batteries.
At present, lithium hexafluorophosphate (LiPF6) is used as a conducting salt in all commercial lithium ion batteries. This salt has the necessary prerequisites for use in high-energy cells, i.e. it is easily soluble in aprotic solvents, it leads to electrolytes having high conductivities, and it has a high level of electrochemical stability. Oxidative decomposition first occurs at potentials of  greater than approximately 4.5V. LiPF6, however, also has serious disadvantages, which are mainly to be attributed to its lack of thermal stability. In solution, a dissociation into LiF and PF5 takes place, even if only slight, which can lead to a cationic polymerisation of the solvent, caused by the Lewis acid PF5. Upon contact with moisture, caustic hydrofluoric acid is released, which, on the one hand makes handling more difficult, because of its toxicity and corrosiveness, and, on the other hand, can lead to the (partial) dissolution of the transition-metal oxides (for example LiMn2O4) used as cathode material. In this way, the cycle stability of the respective electrochemical energy store is affected.
With this background in mind, intensive efforts are being made with the aim of developing alternative conducting salts. As such, lithium salts with perfluorated organic radicals are being tested above all. In particular, lithium trifluoromethane sulphonate, lithium bis(trifluoromethane sulphonyl)imide and the lithium methides, the most fundamental of which is lithium bis(trifluoromethane sulphonyl)methide, are to be mentioned. These salts also have disadvantages, which hitherto prevented their use in commercial lithium batteries. The first-mentioned salt does not give the electrolytes produced with it a sufficiently high conductivity. The last-mentioned salts admittedly have a conductivity which is equal to that of LiPF6, but because of the costly production methods are not of interest commercially. Additionally, the imide has a corrosive effect on aluminium sheets, which are used as current diverters in many battery systems. Apart from this, because of the high fluorine content of the compounds, under unfavourable conditions exothermal reactions with lithium are to be feared.
Lithium organoborates were tested as a further class of compound for use as a conducting salt. However, their use in lithium ion batteries was not seriously taken into consideration because of the low oxidation stability, the safety problems linked with the formation of triorganoboranes as well as their high price.
The lithium borate complex salts [(R1O)2B(OR2)2]Li described in DE 19633027 A1 represent a substantial step forward. In this connection, R1 and R2 are the same or different, R1 and R2 are, if appropriate, connected to each other by a single bond or a double bond, R1 and R2 may be, individually or jointly, an aromatic ring from the group phenyl, naphthyl, anthracenyl or phenanthrenyl, which can be unsubstituted or substituted one to four times by A or Hal, Hal standing for fluorine or chlorine and A meaning alkyl with 1 to 8 C-atoms, which in turn can be halogenised one to four times.
A disadvantage of these compounds is, on the one hand, the stabilities of the non-fluorinated derivatives which, although improved, are in no way sufficient for the 3V systems required. Thus, for example, the unsubstituted lithium-bis[1,2-benzenediolato(2-)-O,O{grave over ( )}] borate(1-) decomposes when an anodic potential of 3.6 V is exceeded. This value lies clearly below that of the standard conducting salt LiPF6 (approximately 4.5V). As a result of increasing fluorine substitution of the organic radical, the oxidation stability rises to a value of approximately 4V for the perfluorated compound. However, these values are still lower than in the case of the standard salt LiPF6. The stability of the borates which are described, however, increases further because of a top layer formation during cyclisation, so that for some compounds almost sufficient stabilities are achieved. The stable compounds, however, have high molar masses (for example 378 g/mol for the perfluorated catecholate compound). Also, the preliminary stages required for the synthesis are not commercially available, but instead have to be produced in a costly way. Finally, compounds with CF bonds represent a potential safety risk, because they are not thermodynamically stable with respect to metallic lithium.
The underlying object of the invention is therefore to eliminate the disadvantages of the prior art and to develop an electrochemically stable lithium compound which has a good solubility in the aprotic solvents used by the battery industry, and also a method for the production thereof.
The object is achieved by the lithium compound lithium-bisoxalatoborate, Li[(C2O4)2B], indicated in claim 1. The independent claims 2 and 11 indicate two different methods for the production of lithium-bisoxalatoborate, claims 3 to 10 and 12 to 13 develop the method further and claim 14 indicates a use of the compound lithium-bisoxalatoborate.
Surprisingly, although it does not have any fluorine substituents, lithium-bis(oxalatoborate) has an excellent oxidation resistance. Thus, solutions of this salt in a mixture of ethylene carbonate (EC) and 1,2-dimethoxyethane (DME) are stable up to a voltage of 4.6V.
Furthermore, the conductivities which can be achieved with the salt in accordance with the invention are note worthy. Thus, a 0.56 m solution in a 1:1 mixture of EC and DME has a conductivity of 10.3 mS/cm at room temperature. In the usual solvent mixture propylene carbonate (PC)/DME (1:1), the conductivity of lithium-bisoxalatoborate in the case of different concentrations was measured (FIG. 1). It can be inferred from the measurement results that with concentrations of up to 15% by weight, conductivities of up to 14 mS/cm are achieved (see FIG. 1). These values are at the same level as, or even above, the conductivities which can be achieved with LiPF6. Thus, for 1 m solutions of LiPF6 in dimethyl carbonate (DMC)/EC, 11.0 mS/cm is achieved.
The molar mass of 193.8 g/mol is admittedly approximately 27% above that of the LiPF6, but clearly below that of the borates described in DE 19633027 A1. This is not problematic, however, because electrolytes with lithium-bis(oxalatoborate) are also sufficiently conductive at lower concentrations (for example approximately 0.5 mol/l).
The lithium-bis(oxalatoborate) is easily soluble in water and in many polar aprotic solvents. In tetrahydrofuran (THF), approximately 42% by weight dissolves at 50xc2x0 C. and approximately 30% by weight dissolves at 23xc2x0 C. It has a solubility of at least 15% by weight in diethylene glycol dimethyl ether (diglyme) and mixtures of diglyme and carbonates.
According to thermogravimetry experiments, lithium-bis(oxalatoborate) is fully stable at up to approximately 300xc2x0 C.
The lithium-bis(oxalatoborate) in accordance with the invention can be produced by reacting a lithium compound, such as lithium hydroxide (anhydrous or the hydrate) or lithium carbonate or a lithium alkoxide, with oxalic acid or an oxalate and a boron compound, such as boron oxide or boric acid or a boric acid ester.
The reaction can be carried out in a solvent, but does not necessarily have to be.
Preferably, lithium hydroxide or lithium carbonate is reacted with a stoichiometric amount of oxalic acid and a stoichiometric amount of boric acid or boron oxide in water, for example: 
The reaction of lithium oxalate with oxalic acid and boric acid or boron oxide in water is also preferred, for example: 
The sequence in which the components are added does not play a significant role. Preferably, oxalic acid is placed in an aqueous solution and the calculated amount of lithium base is added, or lithium oxalate is mixed with the 3-fold molar amount of oxalic acid. Subsequently, the calculated amount of boric acid or boron oxide is added to this partially neutralised oxalic acid solution.
The reaction temperature lies in the range between 0 and 100xc2x0 C.
After the end of dosing, the mixture is heated to 50 to 100xc2x0 C. for a time and the water is then distilled off. When crystallisation begins, the pressure is slowly lowered. The final drying takes place whilst stirring, at approximately 50 to 150xc2x0 C. and  less than approximately 1 mbar.
A solid product is obtainable which is partially lumpy, granular or fine-crystalline solid depending on the drying unit which is selected.
In a variant of the production method in accordance with the invention, water is not necessarily added as the solvent. However, water forms as a reaction by-product in different amounts. According to this variant of the method, it is provided that the starting materials are suspended in an organic solvent and the water which is released during the formation reaction is removed by azeotropic distillation. All solvents which cannot be mixed with water or which can be mixed therewith to a limited extent, which form a water/solvent azeotrope and have such a high volatility that a subsequent product drying is possible, are suitable for this process. Depending on the temperature and stirring conditions selected, the reaction starts spontaneously or is initiated by the addition of small amounts of water. The reaction temperature of the exothermic reaction lies between 0 and 150xc2x0 C. The reaction mixture is subsequently heated to boiling temperature, the water of crystallisation and reaction water being removed by azeotropic distillation. Aromatic substances, such as benzene, toluene, xylene and ethyl benzene, are particularly suitable for the course of the reaction and the azeotropic dehydration. Thus, for example, when toluene is used, the calculated amount of water can be precipitated within a reaction of time of approximately 2 to 4 hours.
The product in accordance with the invention precipitates in fine-crystalline, free-flowing form, completely anhydrous and with good purity. It is separated from the reaction solvent by filtration, washed with an aprotic solvent (for example toluene or comparatively volatile hydrocarbons, such as hexane or pentane) and dried in a vacuum and/or at comparatively high temperatures (50 to 150xc2x0 C.).
Ethers which cannot be mixed with water, such as 2-methyl tetrahydrofuran, for example, are also suitable to a limited extent. In ethereal solvents, however, the lithium-bisoxalatoborate is only formed in impure form, i.e. it subsequently has to be purified in a relatively costly way by fractional crystallisation.
According to a further embodiment of the method in accordance with the invention, the product in accordance with the invention can also be obtained starting from lithium alkoxides LiOR and boric acid esters B(OR)3 (with R=methyl, ethyl). In order to do this, a lithium alkoxide is mixed with a boric acid ester, the corresponding lithium tetraalkoxy borate Li[B(OR)4] presumably being formed. This reaction does not necessarily require a solvent, but can be carried out in the presence of a solvent. The reaction mixture is subsequently reacted with oxalic acid and the alcohol component which is released is removed by distillation. Ideally, those boric acid esters which release as much volatile alcohols as possible are taken for this variant, i.e. the methyl compound or ethyl compound: 
The alcohol itself (i.e. methanol or ethanol) or an aprotic solvent, such as acetonitrile, can be used as the solvent. In this variant of the method, the reaction temperature amounts to 0 to 100xc2x0 C., the range between approximately 20 and 70xc2x0 C. being most suitable. When acetonitrile is used, then, after distillation of the alcohol which is released at normal or reduced pressure, the product in accordance with the invention precipitates upon cooling, in the form of colourless crystals, which can be filtered off and cleaned by washing with acetonitrile or another volatile, aprotic solvent (for example hexane, pentane, diethyl ether).
In accordance with a further variant of the method, LiBO2 as both lithium compound and boron compound can be reacted together with oxalic acid to form the desired product: 
In accordance with a further production method in accordance with the invention, lithium-bis(oxalatoborate) can also be prepared in aprotic media directly in fully anhydrous form. In order to do this, lithium boro-hydride is reacted in a solvent in accordance with the following reaction equation with two equivalents of anhydrous oxalic acid: 
The reaction is advantageously carried out in a solvent in which LiBH4 has a certain solubility, for example in ethers such as tetrahydrofuran (THF). Particularly advantageously, those solvents which are commonly used by the battery industry for the production of electrolytes are also used. In particular, polyethers, such as 1,2-dimethoxyethane, are suitable. The reaction temperature is not of crucial importance. It is limited downwards by the viscosity, which rises as the temperature falls. On the other hand, however, it should not rise too high, in order to avoid a reductive attack, possible in principle, of the hydride on the oxalic acid or lithium-bis(oxalatoborate). In general, the temperature range between 20 and 50xc2x0 C. is most suitable. The course of the reaction can be followed simply by observing the formation of gas.